Everyone keep flogging the idea that the monitor needs calibrating. Sure, that's right. But that is not what OP is having the problem with. He wants to know why the image looks good on the monitor without post processing adjustment. One explanation that is also being heavily pushed hereis that the image was underexposed, then too bright a monitor makes it look right. But what I said above makes more sense. If you were a camera design engineer, how would you set up your JPEG algorithm. Considering almost everyone has a too-bright monitor, and many, many folks don't post process, and lots of folks just send images from computer or phone to another of the same, wouldn't you tone down your JPEGs so they look good on the common monitor? Then these would print dark at home, but many print labs would fix the problem. That is what we have here in my opinion.

Yes, if OP calibrates his monitor, he will see that the JPEGs are too dark for any reason, and he will have to fix them. Or he can output RAW, but he will have to PP everyone of those too. If he prefers JPEG, he can adjust in camera to just put out a brighter JPEG and not have to post process, but then only a calibrated monitor will show the images correctly. That is not the same thing as exposing more; it is just using a different portion of the captured image brightness range.